/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    int maxDepth(TreeNode* root) {
        if (root == nullptr)return 0;
        vector<TreeNode*>q;
        int ans = 0;
        q.push_back(root);
        while (q.size()) {
            vector<TreeNode*>q1;
            ans++;
            for (auto x : q) {
                if (x->left)q1.push_back(x->left);
                if (x->right)q1.push_back(x->right);
            }
            q = move(q1);
        }
        return ans;
    }
};